The invention is directed to a crucible for containing salt baths used for the boriding of steels. The boriding of steel is a relatively young process whose significance has grown considerably in recent years. For boriding, the steel parts are immersed in boron supplying salt melts and annealed for several hours at 850.degree.-950.degree. C. Thereby there is built up a hard, wear resistant layer of iron boride (Fe.sub.2 B) through diffusion of boron into the steel surface.
The boriding salt melts can be operated in the air and it is sufficient for purpose of the treatment to immerse in the melt the parts secured on the charge frame, withdraw the parts after completing the boriding, quenching in water and rinse off the salt residues.
However, great industrial use of this simple boriding process thus far has been retarded by the considerable corrosion problems with crucibles used for containing the salt baths. Generally the boriding salt melts consist of a mixture of alkali and alkaline earth metal chlorides, e.g. sodium chloride, potassium chloride, barium chloride or calcium chloride, alkali and alkaline earth metal fluorides, e.g. sodium fluoride, potassium fluoride, barium fluoride or calcium fluoride, boron oxide and borates, e.g. borax or sodium metaborate, as well as a boriding agent, for the most part boron carbide or boron powder. These melts are operated at 850.degree.-950.degree. C. Thereby especially the fluorides and the borates very quickly cause corrosion of crucibles made of normal steels. Thus customary crucibles of carbon steel or low alloyed steels, as are customarily used in the hardening art, only withstand the corrosion attack of the boriding salt melt for several weeks. In this case, in the first place the corrosion depends on an oxidative corrosion of the crucible material which corrosion is greatly accelerated by the oxide dissolving and corroding action of the fluoride and borate. Therefore there have already been attempts to use as the material for the crucible corrosion resistant, high nickel containing special alloys based on nickel-chromium having nickel contents of 50-90 wt. % and chromium contents of 13-23 wt. % as well as small amounts of aluminum, cobalt, iron, manganese, titanium, tungsten, and molybdenum or even of pure nickel.
However, these materials are likewise quickly corroded by boriding salt melts. High alloyed, corrosion resistant steels, such as the known VA-steels, likewise are unusable as crucible material. They are corroded both by the attack of the boriding agent and also corroded by the attack of the oxide dissolving borate and fluoride. The titanium crucibles occasionally used in the art are not resistant to fluorides and other materials suitable in princible such as noble metals, tantalum or graphite are not usable for reasons of cost and for industrial reasons.
Therefore it was the problem of the present invention to provide a crucible for containing salt baths for the boriding of steels which are only slightly attatcked by the salt baths and have high "on-stream" times.
Unless otherwise indicated all parts and percentages are by weight.